A typical wafer mounting arrangement includes a backplane for supporting a wafer during wafer processing and a clamping device called a wafer holder. For a number of wafer processes, including sputter deposition, the wafer must be heated while mounted to the backplane. Prior wafer processing devices have employed one or more of a number of different types of wafer heating mechanisms for heating the backplane and/or the wafer, including resistive heating, RF induction heating, backside gas heat conduction, radiant heat, etc. This invention relates to the use of a resistive element to heat the backplane, thereby indirectly heating a wafer mounted thereon.
For most wafer coating processes, including sputter deposition, to obtain uniformity in the sputtered film of particles, the temperature of the wafer must be uniform across its entire surface area. This requires consistent heat transfer between the heating element and the backplane.
Effective heat transfer between the backplane and a resistive element depends upon maintaining good surface contact between the resistive element and the backplane. If the resistive element is shaped to be embedded within a correspondingly shaped void in the backplane, machining of both components must be very precise. This precise machining can be relatively expensive. Moreover, even if a precise fit is achieved, over a period of use, some radial expansion and contraction of either the backplane or the resistive element caused by heating and cooling may adversely affect the precise and tightly machined fit.
As an alternative to embedding the resistive element to the backplane, the resistive element may be secured to the backplane by one or more bolts. This allows the resistive element to be tightened into good contact with the backplane at the beginning of each period of use. However, most resistive elements of this type are annular in shape. As a result, tightening of one of the bolts invariably results in some loosening of the adjacent bolts. Thus, it is difficult to achieve consistent surface to surface contact between the annular resistive heating element and the backplane around the entire circumference of the annular heating element.
In short, it is difficult and/or costly to maintain good solid to solid surface contact between a resistive heating element and a backplane to assure effective heat transfer therebetween and uniform heating of a wafer.
For a wafer heating chuck with an annular resistive heating element contacting a backside of the backplane, heat transferred to the backplane will drop off radially outside of the resistive heating element. As a result of the heating chuck being mounted to cooler parts, the wafer temperature at the peripheral edge of the wafer will be lower than the wafer temperature at the center or midportion of the wafer.
One possible solution to this problem of temperature drop-off adjacent the wafer perimeter involves using backplane and a single resistive element larger than the wafer diameter and to contact the backplane with the heating element below the entire surface area of the wafer. However, many existing wafer heating chucks have only a limited amount of space for locating the heater behind the backplane and clamping it thereto, due to the space requirements for additional components such as cooling channels, mounting bolts, gas inlets etc. Thus, unless one were to redesign the entire process chamber and wafer holder, this solution is not workable.
Even if a single resistive heating element could be employed to heat the entire back surface of an oversized backplane, this would not guarantee temperature uniformity across the wafer. Heat generated in the processing chamber by the process itself also adds heat to the backplane, usually in a non-uniform manner. For example, during cathode sputtering, because the target and the wafer are located quite close, the ionized plasma confined by a magnetic field adjacent the target surface generates heat at both the target and the wafer. To maximize utilization of target material, it is common to provide multiple erosion zones by using more than one magnetic field and/or to move one plasma-confining magnetic field, thereby moving the plasma and changing the location of the process generated heat. The presence of one or more annular zones of process-generated heat further complicates attempts to achieve uniformity in wafer temperature.
It is an objective of the invention to assure solid to solid surface contact between a resistive heating element and a backplane over the lifetime of the heating element and the backplane.
It is another objective of the invention to more effectively heat the perimeter of a backplane without requiring substantial redesign of the entire process chamber and wafer holder.
It is still another objective of this invention to achieve uniformity in wafer temperature across the surface area of a wafer during sputter deposition thereon, despite heat generated by a sputter plasma.
The above-stated objectives are met by a wafer heating chuck which employs a combination of features, including a wedge-shaped outer heater shaped to reside in a wedge-shaped recess in the backplane, so that good surface contact therebetween may be obtained by applying only a minimal clamping force. The wedge shape of the heater also enables more electrical heat to be dissipated into the outer portion of the backplane, where heat loss is greatest.
The invention also employs a clamping member for separately clamping the inner and outer heaters to the backplane to assure intimate solid to solid contact therebetween. In one embodiment, the clamping member includes separately cantilevered tabs which enable the inner heater to be firmly clamped to the backplane around its entire circumference. In an alternative embodiment, two sets of cantilevered tabs enable both the inner and outer heaters to be firmly clamped to the backplane around its circumference. The cantilevered tabs also flex to ensure positive contact during thermal expansion and contraction. The two heaters are independently controllable, so that the outer heater can be used effectively to eliminate heat loss at the perimeter of the backplane.
In addition to the two heaters and the clamping member, the invention includes a pair of sensors for sensing temperature at two spaced regions of the backplane located adjacent the inner and outer heaters. An electrical controller connected to the heaters and the sensors allows the backplane temperature to be profiled to compensate for process heat, such as localized heating caused by a confined plasma during cathode sputtering. The heat added to the wafer by the wafer heating chuck components is concentrated in those areas which are not heated by process generated heat. As a result, the composite heat generated by both sources results in substantial temperature uniformity across the wafer.
Additionally, the backplane includes a recess in communication with a gas inlet to supply a backside gas between the wafer and backplane, thereby to speed up heating and cooling of the wafer.
According to one embodiment of the invention, a wafer heating chuck includes a backplane, two resistive heaters, a clamping member, two temperature sensors, a gas inlet, and inner and outer cooling channels. The backplane includes a front surface for supporting a wafer thereon. The front surface includes a recess supplied with a backside gas via a gas inlet located in the center of the backplane. The recess includes three spokes which extend radially outwardly from the center and a circumferential ring which interconnects the outer ends of the radial spoke portions adjacent the peripheral edge of the backplane.
A second or rearward surface of the backplane includes a recess in an outer region thereof, adjacent the peripheral edge of the backplane. The recess includes an outer angled wall. An outer annular heater resides within the recess, and the outer heater includes an angled surface which is complementary with respect to the angle of the recess wall. An inner annular heater resides radially on the backplane inside the outer heater. A clamping member secures to the second surface of the backplane, as by bolts, and the clamping member separately clamps the outer and inner heaters to the second surface of the backplane to assure good solid to solid contact for optimum heat transfer therebetween. A plurality of outer threadable retainers supported by the clamping member enable the outer annular heater to be clamped to the backplane with a force which may be set independently of the bolts which hold the clamping member to the backplane.
Additionally, a plurality of inner threadable retainers enable the inner annular heater to be clamped to the backplane with a force which is also independent of the securing bolts. Moreover, each of the inner retainers is supported by an inner radially cantilevered, trapezoid-shaped tab so that tightening of one of the inner retainers does not adversely affect the clamping force applied to the inner heater by adjacent inner retainers. Stated another way, when threadably torquing an inner retainer against the inner annular heater, a flexing force which is applied to the clamping member in the opposite direction, i.e. away from the backplane, acts only on a single tab, so that each inner retainer and associated tabs may be set and maintained at a predetermined clamping force independent of the other inner retainers and tabs.
In an alternative embodiment of the invention, the clamping member includes outer circumferentially cantilevered tabs which support the outer threaded retainers. Similar to the inner tabs, the outer tabs allow tightening of one of the outer retainers without adversely affecting the clamping force applied by adjacent outer retainers. Therefore, both inner and outer heaters and their contact to the backplane may be adjusted in segments around the backplane.
An inner heat sensor, preferably a thermocouple, mounts to the second surface of the backplane, between the inner annular heater and the gas inlet. An outer heat sensor, again preferably a thermocouple, mounts to the second surface of the backplane outside of the recess. The inner and outer sensors detect the temperature of the backplane at inner and outer regions thereof, respectively. The backplane further includes internal cooling channels supplied with gaseous or liquid coolant via inlet and outlet conduits. The wafer heating chuck also has an outer cooling ring or plate with outer cooling channels supplied with inlet and outlet conduits for circulation of a liquid coolant.
With inner and outer independently controllable heaters, the temperature of the inner and outer regions of the front surface of the backplane may be independently controlled. Because of the shape of the outer annular heater and its location within a recess in the backplane, and the configuration and operation of the segmented clamping member with respect to both heaters, the wafer heating chuck of the present invention assures consistently good solid to solid contact between the heaters and the backplane, thereby maximizing heat transfer therebetween. Additionally, use of backside gas enables this heat to be more quickly transferred between the backplane and the wafer mounted thereon, thereby reducing the time necessary to heat the wafer to an initial process temperature necessary to begin sputtering.
In one embodiment of the present invention, the resistive heating elements utilized for the inner and outer heater each includes a miniature tubular sheath surrounding two generally parallel wire elements and insulative material which provide the necessary resistance for the heaters. An alternative embodiment utilizes a single tubular sheath surrounding a coil and insulative material which provides resistance and subsequent heat for the heaters.
With independent control of each of the heaters, and by using feedback from the inner and outer temperature sensors to obtain some indication of the magnitude of process generated heat, the heat transferred to the backplane can be profiled in accordance with the temperature profile on the wafer induced by a plasma, thereby to produce a flat composite temperature profile across the wafer, indicating temperature uniformity across the surface area.